This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-199999 filed on Jun. 29, 2001.
1. Field of the Invention
The present invention relates to a dynamo-electric machine, such as a blower motor of a vehicle air conditioning system.
2. Description of Related Art
A motor, such as a blower motor of a vehicle air conditioning system, includes a yoke. A pair of curved magnets, which are curved to conform with an inner circumferential surface of the yoke, are secured to the inner circumferential surface of the yoke in diametrically opposed relationship to each other.
A rotor is rotatably received within the yoke. A drive shaft extends along a rotational axis of the rotor and is secured to the rotor to rotate integrally with the rotor. A predetermined number of cores are circumferentially arranged in the rotor at equal angular intervals. Each core has a rectangular plate tooth, which radially outwardly protrudes toward the yoke. A winding is wound around each tooth.
The blower motor of the vehicle air conditioning system is normally placed in a passenger cabin of a vehicle, so that the blower motor is required to achieve a relatively high degree of silence during rotation of the blower motor. Thus, rather than using ball bearings, which can relatively easily transmit vibrations from the rotor (armature), slide bearings made of a sintered metal material, which does not easily transmit vibrations from the rotor (armature), are commonly used to rotatably support the drive shaft of the blower motor of the vehicle air-conditioning system.
However, the slide bearing requires oil in a contacting portion, which makes sliding contact with the drive shaft of the motor. Thus, maintenance of the oil is required, and spill of the oil from the slide bearing could occur.
Furthermore, in the slide bearing, a washer or the like is required, causing an increase in the number of the components. This results in an increased complexity of the bearing structure of the motor and an increased complexity of the manufacturing steps of the bearing structure of the motor.
On the other hand, unlike the slide bearing, if the ball bearing is used in the bearing structure of the motor, the oil in the contacting portion, which makes sliding contact with the drive shaft of the motor, is not required. Thus, with use of the ball bearing, the disadvantages, which result from the maintenance of the oil or the spill of the oil, can be avoided, and the washers or the like are not required. This substantially simplifies the bearing structure of the motor and the manufacturing steps of the bearing structure of the motor.
The ball bearing can relatively easily transmit vibrations induced, particularly, by cogging torque of the rotor (i.e., torque generated in the rotor due to changes in attractive force and repulsive force between the magnets and the rotor). Thus, it is effective to reduce the cogging torque to reduce the vibrations of the rotor and to achieve a higher degree of silence during the rotation of the motor.
One way of reducing the cogging torque is to gradually reduce a wall thickness (radial dimension) of each magnet toward its circumferential ends. However, this measurement alone cannot reduce the cogging torque to an acceptable level, which allows use of the ball bearing in the blower motor of the vehicle air conditioning system.
Another way of reducing the cogging torque is to use a rotor having skewed cores. Although this measurement can reduce the cogging torque, it generally results in some disadvantages, such as a reduction in a winding surface area of each core, around which the winding is wound.
The present invention addresses the above disadvantages. Thus, it is an objective of the present invention to provide a dynamo-electric machine, which effectively reduces cogging torque.
To achieve the objective of the present invention, there is provided a dynamo-electric machine, which includes a yoke, a rotor and a plurality of curved magnets. The rotor is rotatably received in the yoke and includes a predetermined number of cores circumferentially arranged at substantially equal angular intervals, and each core includes a tooth, which radially outwardly extends toward the yoke. The magnets are secured to an inner circumferential surface of the yoke. Each magnet has first and second tapered portions, which are tapered in opposite circumferential directions. Each of the first and second tapered portions has axially opposed first and second tapered surfaces. An axial distance between the first tapered surface and the second tapered surface of each of the first and second tapered portions decreases toward an outer circumferential end of each of the first and second tapered portions. The first tapered surface of each of the first and second tapered portions has a first opposing point, which is radially opposed to a first imaginary end circle defined by outer peripheral surfaces of the cores at one axial ends of the cores. The second tapered surface of each of the first and second tapered portions has a second opposing point, which is radially opposed to a second imaginary end circle defined by the outer peripheral surfaces of the cores at the other axial ends of the cores. The rotor and each magnet are configured to satisfy one of the following conditions: (Dxcfx80/Z)Xn+T less than X less than (Dxcfx80/Z)xc3x97(n+1)xe2x88x92T and (Dxcfx80/Z)xc3x97(nxe2x88x921)+T less than X less than (Dxcfx80/Z)Xnxe2x88x92T, wherein Z is a number of the cores of the rotor, D is an outer diameter of the rotor, T is a circumferential dimension of each tooth, X is a circumferential dimension between a circumferentially innermost one of the first and second opposing points of the first tapered portion and a circumferentially innermost one of the first and second opposing points of the second tapered portion when each magnet is viewed from a radial direction, and n is the number of the teeth that are entirely placed within a range defined by X when a circumferential center of one of the teeth is radially opposed to a circumferential center of the range defined by X.